Dimensional Isomerism in COFs Enabled by Spirobifluorene Substitution for Enhanced Photocatalytic CO 2 Reduction

ABSTRACT The concept of dimensional isomerism—covalent organic frameworks (COFs) constructed from identical molecular precursors but forming structures of different dimensions—remains largely unexplored. Here, we report a pair of isomeric COFs derived from the same spirobifluorene core, wherein substitution at either the C2/C7 or C3/C6 positions leads to the formation of a planar two‐dimensional (2D) or a nonplanar three‐dimensional (3D) framework, respectively. This substitution‐controlled strategy enables precise modulation of π‐conjugation and framework dimensionality without altering the molecular identity of the building block. Comparative photocatalytic experiments reveal that the 2D C2/C7‐substituted COF exhibits significantly superior CO 2 reduction activity under visible light, attributed to its extended conjugation and efficient charge transport. These findings underscore the critical role of molecular connectivity in determining the structure–function relationships of COFs and propose a generalizable approach for designing high‐performance photocatalytic materials through topological isomerism.